1. Field of the Invention
The present invention relates to a timing estimator of an OQPSK demodulator applied to a Zigbee receiver, and more particularly, to a timing estimator of an OQPSK demodulator that can improve degradation in receiving performance due to a frequency error by obtaining a phase difference between a conjugate complex number signal of a reception signal delayed by a set time and the received signal to offset the frequency error of the reception signal.
2. Description of the Related Art
Recently, as Ubiquitous that means a communication environment in which users can access to a network whenever and wherever they wish without a limitation in time and place is proposed, researches on the small scale wireless communication system, for example, wireless local area network (WLAN), wireless personal area network (WPAN), sensor network, RFID and the like, not on the large scale communication network, for example, cellular network, are being actively performed.
Particularly, in the field of WPAN or/and sensor network among these wireless communication systems, main requirements are an ultra miniaturization, low power consumption and low price. However, the wireless communication system, such as WPAN and the like, has a difficulty in employing the high performance and high price equipments or components used in the related art cellular wireless communication system or WLAN system without any change. To the contrary, when the wireless communication system uses low price components so as to lower the price, since a large frequency error or phase error that cannot be ignored may be generated, it is necessary to make up some counterplan for such errors.
Considering these circumstances, it is, in the communication system environment, required to research and develop a demodulator that can show an excellent performance without using the high price equipment or component. Especially, in a receiver receiving the OQPSK symbol packet, it is required to perform the symbol coherency more accurately.
The OQPSK demodulator includes a timing estimator so as to demodulate a symbol from a received signal. The related art timing estimator will now be described with reference to FIG. 1.
FIG. 1 is a schematic view of a timing estimator of an OQPSK demodulator according to the related art.
Referring to FIG. 1, the timing estimator includes a frequency-coherent/phase-coherent processing section 5 processing frequency coherency and phase coherency for an RF signal or IF signal, an A/D converter (ADC) 10 converting an analog reception signal ‘Ir, Qr’ of the frequency-coherent/phase-coherent processing section 5 into a digital reception signal, a correlation operating section 20 performs a correlation operation between an output signal of the A/D converter 10 and a reference symbol to obtain correlation values, and a coherency detector 30 determining a detection point when the largest correlation value is obtained by the correlation operating section 20 as a coherent point and outputting a coherent signal.
The correlation operating section 20 includes a first matched filter 21 correlating a real number part, ‘r{r(k)}’ of the reception signal of the A/D converter 10 with a real number part ‘r{s(k)}’ of the reference symbol, a second matched filter 22 correlating an imaginary number part ‘im{r(k)}’ of the reception signal of the A/D converter 10 with an imaginary number part ‘im{s(k)}’ of the reference symbol, and an adder 23 adding the correlation values of the first and second matched filters 21 and 22.
At this time, when the reception signal ‘r(k)’ from the A/D converter 10 and the reference symbol ‘s(k)’ are respectively defined as shown in equation 1, output signals ‘S1 and S2’ of the A/D converter 10 are expressed as the below equation 2. Also, when the real number part ‘S3(Re{s(k)})’ and the imaginary number part ‘S4(im{s(k)})’ of the reference symbol respectively inputted into the first matched filter 21 and the second matched filter 22 are expressed by the below equation 3, output signals ‘S5, S6’ of the first and second matched filters 21 and 22 are expressed as shown in equation 4.r(k)=ej{θ(k)+Φ(k)+2πΔf0k},  Equation 1:
where Δf0=f0Ts, Ts=sampling period, Φ(k)=phase error, f0=frequency error, and k=digital time index; ands(k)=ejθk.
      Equation    ⁢                  ⁢    2    ⁢          :                          S        ⁢                                  ⁢        1            =              cos        ⁡                  (                                    θ              ⁡                              (                k                )                                      +                          Φ              ⁡                              (                k                )                                      +                          2              ⁢              πΔ              ⁢                                                          ⁢                              f                0                            ⁢              k                                )                      ;    and              S      ⁢                          ⁢      2        =                            sin          ⁡                      (                                          θ                ⁡                                  (                  k                  )                                            +                              Φ                ⁡                                  (                  k                  )                                            +                              2                ⁢                πΔ                ⁢                                                                  ⁢                                  f                  0                                ⁢                k                                      )                          .                                  ⁢        Equation            ⁢                          ⁢      3      ⁢              :                                S        ⁢                                  ⁢        3            =              cos        ⁢                                  ⁢                  θ          ⁡                      (            k            )                                ;    and              S      ⁢                          ⁢      4        =          sin      ⁢                          ⁢              θ        ⁡                  (          k          )                          Equation    ⁢                  ⁢    4    ⁢          :                          S        ⁢                                  ⁢        5            =                        ∑                      k            =            1                    N                ⁢                              cos            ⁡                          (                                                θ                  ⁡                                      (                    k                    )                                                  +                                  Φ                  ⁡                                      (                    k                    )                                                  +                                  2                  ⁢                  πΔ                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  k                                            )                                ⁢          cos          ⁢                                          ⁢                      θ            ⁡                          (              k              )                                            ;    and              S      ⁢                          ⁢      6        =                  ∑                  k          =          1                N            ⁢                        sin          ⁡                      (                                          θ                ⁡                                  (                  k                  )                                            +                              Φ                ⁡                                  (                  k                  )                                            +                              2                ⁢                πΔ                ⁢                                                                  ⁢                                  f                  0                                ⁢                k                                      )                          ⁢        sin        ⁢                                  ⁢                              θ            ⁡                          (              k              )                                .                    
Also, an output signal ‘S7’ of the adder 23 is expressed by the below equation 5, and can be converted in brief into the below equation 6 when Φ(k)=0 and 2πΔf0k=0 in the equation 5.
      Equation    ⁢                  ⁢    5    ⁢          :                  S      ⁢                          ⁢      7        =                  ∑                  k          =          1                N            ⁢                                    (                                                                                                                              cos                        ⁡                                                  (                                                                                    θ                              ⁡                                                              (                                k                                )                                                                                      +                                                          Φ                              ⁡                                                              (                                k                                )                                                                                      +                                                          2                              ⁢                              πΔ                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              k                                                                                )                                                                    ⁢                      cos                      ⁢                                                                                          ⁢                                              θ                        ⁡                                                  (                          k                          )                                                                                      +                                                                                                                                          sin                      ⁡                                              (                                                                              θ                            ⁡                                                          (                              k                              )                                                                                +                                                      Φ                            ⁡                                                          (                              k                              )                                                                                +                                                      2                            ⁢                            πΔ                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            k                                                                          )                                                              ⁢                    sin                    ⁢                                                                                  ⁢                                          θ                      ⁡                                              (                        k                        )                                                                                                                  )                    .                                          ⁢          Equation                ⁢                                  ⁢        6        ⁢                  :                                                                            S              ⁢                                                          ⁢              7                        =                        ⁢                                                            ∑                                      k                    =                    1                                    N                                ⁢                                  cos                  ⁢                                                                          ⁢                                      θ                    ⁡                                          (                      k                      )                                                        ⁢                  cos                  ⁢                                                                          ⁢                                      θ                    ⁡                                          (                      k                      )                                                                                  +                              sin                ⁢                                                                  ⁢                                  θ                  ⁡                                      (                    k                    )                                                  ⁢                sin                ⁢                                                                  ⁢                                  θ                  ⁡                                      (                    k                    )                                                                                )                                              =                    ⁢                      maximum            ⁢                                                  ⁢            correlation            ⁢                                                  ⁢                          value              .                                          
However, in the related art OQPSK or demodulation method, when the final correlation output ‘S7’ has a phase error or frequency error component, a correlation result value is very low due to influence of the error even at a timing when the coherency is consistent, so that an exact coherency cannot be found.
So, in the case of PSK signals according to the related art, coherent demodulation is generally performed. The coherent demodulation indicates a method in which the correlation operation is performed after an RF signal received at a front side of the A/D converter is exactly consistent with a coherency of a local oscillator (L.O) of a receiver to completely remove the frequency error and phase error contained in the r(k).
However, since the related art method shows a good performance but has a very complicated system to remove the frequency error and phase error, it is not suitable for the application to a lower power and micro system.